Conventionally, it has been widely practiced to prevent the forgery of cards and security notes by using the system of affixing a hologram onto the object, and visually verifying it. It has also been proposed to use a hologram having a prescribed diffractive property to verify the authenticity of the article. To prevent the uncertainty associated with the visual identification, a light beam of a prescribed wavelength may be impinged upon the hologram so that the authenticity of the article may be verified by detecting the direction of the diffracted light. For such an identification system, reference should be made to U.S. Pat. No. 5,200,794 assigned to the common assignee. The contents of this prior patent is hereby incorporated in this application by reference.
However, due to the recent popularization of the technology of preparing hologram, the hologram technology has become so readily available that illicit duplication of hologram which is hardly distinguishable from an authentic hologram can now be made without any significant difficulty. In other words, the hologram has become less effective in discouraging illicit duplication. Other technologies for preventing forgery are known, but are so costly that they are not suitable for use on common commercial goods. Thus, there is a need for a novel technology for preventing forgery.
Cholesteric liquid crystals normally have a layered structure, and the axial direction of the molecules in each layer are parallel to each other as well to the plane of each layer. Each layer is slightly twisted relative to the adjacent layer so that a three-dimensional spiral structure is produced. This structure demonstrates the property to selectively reflect a circularly polarized light having a wavelength of .lambda. which is given by .lambda.=n.multidot.p where p is the depth of the layers for this axial direction to turn 360 degrees or the pitch, and n is the average index of refraction of each layer. Therefore, if the direction of the liquid crystals in each layer turns counter-clockwise with respect to the incident light, the left-handed circularly polarized component of the incident light having the wavelength of .lambda. is reflected while the right-handed circularly polarized component passes through. The light having any other wavelength passes through. For instance, when a cholesteric liquid crystal material having a property to reflect red light having the wavelength of .lambda..sub.R is placed on a material which absorbs light in the visible range, and a random light such as sunlight is radiated thereon, the transmitted light is all absorbed, and only a left-handed circularly polarized light having the wavelength of .lambda..sub.R is reflected so that the cholesteric liquid crystal material emits bright red light.
A cholesteric liquid crystal material has the property to change color depending on the viewing angle. When the incident angle to the plane of the cholesteric liquid crystal material is .theta., the difference in the length of the light path between the upper surface and the bottom surface of each layer or for each pitch is 2p.multidot.cos.theta.. If this difference is a multiple of the wavelength .lambda. (2p.multidot.cos.theta.=n.lambda. where n is an integer), the components of the light reflected by the upper surface and the bottom surface of each layer reinforce each other. Therefore, as the incident angle gets shallower, a light component having a progressively shorter wavelength is amplified, and the reflected light turns from red to blue.
A cholesteric liquid crystal material having a low molecular weight tends to be sensitive to temperature, an electric field and a magnetic field, and often changes color when affected by such influences. However, a cholesteric liquid crystal material having a high molecular weight is highly stable, and is much less affected by such influences. A high-polymer cholesteric liquid crystal material which has a desired layer pitch and a high level of stability can be obtained by coating a liquid crystal material over PET film, and applying a temperature, an electric field, a magnetic field, a pressure, and additives.
Hologram, which can be given with a prescribed optical property according to the spacing of grooves formed on a surface plane, can be relatively easily duplicated only if a technology for forming fine groove is available. On the other hand, as the optical property of a high-polymer cholesteric liquid crystal material is determined by a three dimensional arrangement of molecules, it is much more difficult to duplicate as it requires a high level of technology for preparing the material, and properly processing it. Therefore, by using such properties of the high-polymer cholesteric liquid crystal material for identification, it is possible to improve the identifiability and prevent the forgery of cards, passports, security notes, gift certificates and so on.
Hologram typically comprises a hologram forming layer which has diffractive grating of various pitches formed thereon and a metallic reflective layer on one side thereof. The light reflected by this reflective layer diffracts into various different directions depending on the pitches thereof so that a different view is created depending on the viewing angle. Typically, the hologram changes color, and shows a three-dimensional view as the viewing angle changes. According to the present invention, the identifiability is enhanced by forming the reflective layer of the hologram with a cholesteric liquid crystal material, and taking advantage of the optical properties of the hologram and the cholesteric liquid crystal material.